Related fields include optical measurement of surfaces by projecting structured-light patterns; projectors that project related information along with a main image; projection of images onto real objects; and real-time augmented reality.
Measurement of surface contours, textures, and discontinuous features such as pits or cracks often requires resolution on the order of microns to millimeters. Optical approaches such as laser scanning and structured-light projection are popular for their speed, non-invasiveness, and acceptable precision and accuracy. Objects routinely measured by these methods include (but are not limited to) aircraft and other vehicle bodies, precision mechanical parts, textiles, glass, sheet metal, granular substances such as abrasives and powders, and in-situ archaeological artifacts. The measurements may be part of fabrication, quality assurance and control, or forensic reconstruction of past events. Measurements of parts of the human body are applicable in a widening range of fields including security, medicine and dentistry, fabrication of prosthetics, fitting of apparel, and immersive games.
In many of these applications, quick return of measurement results is crucial to productivity, and sometimes even to human safety. A manufacturing line or medical procedure may need to be halted immediately upon discovery of an unacceptable error. Also, while some applications may use a metrology instrument in one location full-time, others may need to move the capability frequently between locations.
Many optical instruments that produce excellent measurement results in a quiet, protected laboratory are sorely challenged by the shocks, vibrations, temperature ranges and gradients, air currents, moisture, contaminants, and other variables found in factory and field environments. In these places, space is often cramped and the objects to be measured may be awkwardly positioned or in constant motion. Power outlets may be scarce, and trailing cables an unacceptable hazard. Wireless signals may be blocked or suffer from electromagnetic interference.
Typically, metrology results are displayed on a screen connected to, or integrated with, the instrument. If an operator must mark or repair problem areas on the object, looking (or in some cases walking) back and forth between the screen and the target object, or having the results communicated by a second person, consumes time and creates opportunities for mistakes. “See-through” displays, head-mounted or otherwise, alleviate some of these drawbacks. However, they may create parallax errors or obscure peripheral vision too much for safety. Also, if more than one person needs to look at the results, each of them needs a separate display or they need to take turns viewing.
These practical challenges have created a need for a 3D surface metrology instrument that displays the measurement results directly on the surface being measured. Such a display would remove ambiguity during in-situ repair work, could be viewed by several users simultaneously, and would not obscure their vision of other objects. Ideally, the instrument would be portable (e.g., compact, lightweight, and rugged), fast, accurate, versatile, and easy to use.